Daisy chainable sensors and stimulators for implantation in living tissue

ABSTRACT

An implantable sensor/stimulator is connectable to a controller using just two conductors, which two conductors carry both operating power and data (data commands and/or measured data) between the sensor/stimulator and control circuit. Each sensor/stimulator may be serially connected to another sensor/stimulator, again using only two conductors, thereby allowing a &#34;daisy chain&#34; of such sensors/stimulators to be formed. Each sensor/stimulator in the daisy chain is individually addressable by the control circuit. Input data is sent to the sensors over the two conductors using a phase-modulated biphasic modulation scheme, which scheme also provides operating power for each sensor/stimulator connected to the two conductors. Output data is sent from the sensors to the controller over the same two conductors using a pulse-position presence/absence modulation scheme. The data transmission schemes provide a very high signal-to-noise ratio. Each sensor/stimulator includes a power rectifier circuit, a line interface circuit, a state machine controller, at least one sensor that generates an analog output current as a function of a sensed parameter, a low power current-to-frequency converter circuit, and a counter circuit.

FIELD OF THEINVENTION

The present invention relates to implantable medical devices, and moreparticularly to very small implantable sensors and/or stimulators thatmay be serially connected in a daisy-chain using just two conductors.Important aspects of the invention relate to a very-low power rectifiercircuit, line interface circuit, and current-to-frequency convertercircuit that form part of each daisy-chainable implantablesensor/stimulator, which circuits facilitate the powering and operationof the implantable sensor/stimulator using just two conductors which areshared with all other sensors/stimulators in the daisy chain.

BACKGROUND OF THE INVENTION

In the implantable medical device field, a medical device, configured toperform a desired medical function, is implanted in the living tissue ofa patient so that a desired function may be carried out as needed forthe benefit of the patient. Numerous examples of implantable medicaldevices are known in the art, ranging from implantable pacemakers,cochlear stimulators, muscle stimulators, glucose sensors, and the like.

Many implantable medical devices are configured to perform only thestimulation function, i.e., to stimulate on command a prescribed muscletissue in order cause the muscle to contract. An example of a tinyimplantable stimulator is shown, e.g., in U.S. Pat. Nos. 5,324,316 or5,358,514.

Other implantable medical devices are configured to perform only thesensing function, i.e., to sense a particular parameter, e.g., theamount of a specified substance in the blood or tissue of the patient,and to generate an electrical signal indicative of the quantity orconcentration level of the substance sensed. Such electrical signal isthen coupled to a suitable controller, which may or may not beimplantable, and the controller responds to the sensed information in away to enable the medical device to perform its intended function, e.g.,to display and/or record the measurement of the sensed substance. Anexample of an implantable medical device that performs the sensingfunction is shown, e.g., in U.S. Pat. No. 4,671,288.

Still other implantable medical devices are configured to perform boththe sensing and stimulating function. In such instances, the medicaldevice typically includes separate sensing, stimulating and controlcircuits. The sensing circuit senses the presence or absence of aparticular parameter or substance. The control circuit analyzes theinformation sensed by the sensor and determines whether a stimulationcurrent pulse is needed. If a stimulation current pulse is needed, thecontrol circuit directs the stimulating circuit to provide a specifiedstimulation current pulse. A pacemaker is a classic example of animplantable medical device that performs both the sensing function(sensing whether the heart needs to be stimulated and at what rate) andthe stimulating function (stimulating the heart as needed to maintain adesired heart rhythm).

As medical devices have become more sophisticated, there is a continualneed to use more than one sensor. For example, in some instances, morethan one sensor is needed to measure more than one substance orphysiological parameter. In other instances, more than one sensor may beneeded to measure or sense the same substance or physiological parameterat different locations within the patient's body. Similarly, dependingupon the medical application involved, there may be a need to stimulatemuscle tissue at more than one location in the body. One way ofproviding stimulation at multiple locations is to implant separatestimulators at each desired location and then to coordinate theoperation of the stimulators so as to provide a desired result. See,e.g., U.S. Pat. No. 5,571,148.

Whenever multiple sensors and/or multiple stimulators are implanted andare intended to be used in concert to achieve a desired medicalfunction, there is a need to connect or couple such separate multiplesensors/stimulators to a single control circuit or common control point.Sometimes the control function is performed external to the patient, inwhich case the sensors/stimulators are connected to an implantedtelemetry circuit or equivalent; or, alternatively, a telemetry circuitis included as part of each sensor that allows data and commands to besent, transferred, or otherwise coupled across the tissue/skin of thepatient between an external control device and the implantedsensor/stimulator. At other times, the control function is performed byan implantable control circuit, usually connected directly to theimplanted sensors/stimulators. When an implanted control circuit isused, it usually includes a telemetry circuit, or equivalent circuit,that allows the implanted control circuit to communicate with anexternal programmer, thereby allowing the implanted control circuit tobe programmed, or otherwise modified and/or monitored, by the externalprogrammer.

When multiple sensors/stimulators are used, several problems must beaddressed. For example, unless each of the multiple sensors/stimulatorsare connected to a common controller and/or telemetry circuit, eachsensor/stimulator must employ its own telemetry circuit, or equivalentcircuit, that allows it to be monitored and/or controlled. Suchindividual telemetry or communication circuits may add undue complexityto the implanted sensors/stimulators, increasing the size, weight and/orpower consumption of the sensors. What is needed are relatively simplesensors and stimulators that may be implanted at multiple locationswithin the patient, yet operate independent of each other in anefficient and effective manner.

When multiple sensors/stimulators are directly monitored and/orcontrolled by a control circuit, there must be a direct connection,i.e., at least a separate conductor and a return path, for eachsensor/stimulator. If the number of sensors/stimulators is large, thenumber of separate conductors that are required to control and/ormonitor such sensors/stimulators can become unwieldy. The number ofconductors can become especially large and difficult to manage when eachsensor requires more than two conductors, e.g., as when each sensorperforms multiple functions, requiring a separate output conductor foreach function, in addition to conductors to carry power to the sensor.Moreover, the output signal from many sensors, i.e., the signal thatprovides a measure of the parameter or substance being monitored orsensed, is typically a very low level analog signal that cannot betransmitted over very long distances without amplification or buffering.That is, such low level signals are easily corrupted with noise,particularly when the conductors are placed in a very hostileenvironment (e.g., within living tissue, which is equivalent to beingimmersed in salt water). Low level signals in a hostile environmentresult in a signal-to-noise (S/N) ratio that is unacceptably low. Aunacceptably low S/N ratio, in turn, dictates that signal amplificationand/or special buffering circuits be employed. Such signal amplificationand/or buffering, however, disadvantageously require additionalcircuitry, thereby increasing the complexity, size and weight of thedevice, and further require additional operating power. What is clearlyneeded, therefore, are sensors/stimulators that can be readily operatedin a multiple sensor/stimulator configuration, yet require a minimumnumber of conductors to connect the sensors/stimulators to a controlcircuit, and wherein a high S/N ratio can be maintained for data andcommand signals that are transmitted to and from thesensors/stimulators.

SUMMARY OF THE INVENTION

The present invention addresses the above and other needs by providingan implantable medical device, e.g., a sensor/stimulator, that can beconnected to a controller, typically an implantable controller (or animplantable transceiver that is in communication with an externalcontroller), using just two conductors that carry both operating powerand data (data commands and/or measured data) between the device andcontrol circuit. Moreover, a plurality of such devices may be connectedtogether using just two conductors. That is, a first device may beconnected to the controller using just two conductors. Another device,using the same two conductors as are connected to the first device, maythen be connected to the first device, thereby allowing a "daisy chain"of such devices to be formed. Advantageously, each device in the daisychain is individually addressable by the control circuit, and the mannerin which data is transmitted between a given device and the controlcircuit is very immune to noise, thereby providing a very high S/Nratio.

In accordance with one aspect of the invention, the invention comprisesa medical device having an hermetically sealed part containingelectrical circuitry and a non-hermetically sealed part. Thenon-hermetically sealed part includes a first pair of terminals and asecond pair of terminals. The first pair of terminals function as theinput/output terminals for connecting a first medical device to acontroller using just two conductors, one conductor being connected toeach terminal. The second pair of terminals function as connectionterminals for attaching an additional medical device to the firstmedical device. The input/output terminals of an additional medicaldevice may then be connected to the controller by simply connectingrespective conductors between the connection terminals of the medicaldevice already connected to the controller and the input/outputterminals of the additional medical device. In this fashion, adaisy-chain of such medical devices may be formed.

In one embodiment, the first pair of terminals is electrically connectedto the second pair of terminals through feed-through means that make thedesired electrical connection through the hermetically sealed part. Insuch embodiment, the first feed-through means make electrical contactbetween each terminal of the first pair of terminals and a respectiveportion of the electrical circuitry within the hermetically sealed part.Then, second feed-through means make electrical contact with therespective portions of the electrical circuitry within the hermeticallysealed part and the second pair of terminals so that a direct electricalconnection is established between corresponding terminals of the firstand second pair of terminals. Thus, the first pair of terminals, orinput/output terminals, comprises a means for applying electrical powerand data to the electrical circuitry within the hermetically sealedpart, as well as a means for receiving data from the electricalcircuitry within the hermetically sealed part; and the second pair ofterminals, or connection terminals, comprises a means for passing theelectrical power and data received on the first pair of terminals to acorresponding first pair of terminals of another implantable medicaldevice.

In another embodiment, the first pair of terminals may be connected tothe second pair of terminals directly within or on the non-hermeticallysealed part without passing through the hermetically sealed part. Hence,in this embodiment, only one set of feed-through means need be employedto connect the first and second pairs of terminals to the respectiveportions of the electrical circuitry within the hermetically sealedpart.

In either embodiment, it is a feature of the invention to allow aplurality of such implantable medical devices to be daisy-chainedtogether by simply connecting a pair of conductors between the secondpair of terminals of one implantable medical device and the first pairof terminals of another implantable medical device.

In accordance with another aspect of the invention, the medical devicecomprises an implantable sensor/stimulator device adapted forimplantation in living tissue, and particularly adapted to sense adesired physiological parameter or function, e.g., to sense the glucoselevel of a patient, and/or to stimulate selected tissue with anelectrical shock. Such implantable sensor/stimulator includes: (1) acarrier having first and second pads thereon to which first and secondline conductors may be attached; (2) a low power rectifier circuitcarried by the carrier and connected to the first and second pads, withthe rectifier circuit including means for generating an operatingvoltage from biphasic pulses applied across the first and second pads;(3) a line interface circuit carried by the carrier and connected to thefirst and second pads; (4) a sensor that senses a specified parameter orsubstance and generates an analog output signal which varies as afunction of how much of the specified parameter or substance is sensed;(5) a converter circuit that converts the analog output signal from thesensor to a digital sensor signal comprised of a multiplicity ofrespective bits; (6) state machine means that defines address datacorresponding to the implantable sensor/stimulator, and that receivesdetected data from the line interface circuit and determines if thedetected data corresponds to the defined address data of the implantablesensor/stimulator, and if so, responding thereto by applying the digitalsensor signal to the line interface circuit so that it can betransmitted to the first and second line conductors attached to thefirst and second pads; and (7) means for hermetically sealing therectifier circuit, line interface circuit, converter circuit, and statemachine means.

In such embodiment, the line interface circuit includes a detectingmeans for serially detecting whether input biphasic pulses appliedacross the first and second pads are of a first phase or a second phase,a first phase corresponding to a received data bit representing a binary"1", and a second phase corresponding to a received data bitrepresenting a binary "0". In this manner, an input data stream may bereceived by the line interface circuit from biphasic data pulses thatare applied between the first and second line conductors, i.e., betweenthe first and second pads attached to the first and second lineconductors.

The line interface circuit also includes transmission means for seriallyapplying an output pulse, e.g., a monophasic or preferably biphasicpulse, having a first or second amplitude across the first and secondpads at a time in-between when the input biphasic pulses are appliedacross the first and second pads, where a first amplitude, e.g., amaximum amplitude, of the output pulse represents a binary "1", and asecond amplitude, e.g., a minimum or even a zero amplitude (i.e., theabsence of a pulse) of the output pulse represents a binary "0". In thisway, an output data stream may be transmitted by the line interfacecircuit onto the first and second pads, and hence, onto the first andsecond line conductors attached to the first and second pads.

Advantageously, by using biphasic pulses in the manner described, suchpulses serve a dual purpose:

(1) the energy contained therein may be rectified by the rectifiercircuit and used to power the device, and

(2) the information contained therein may be detected and provide astream of input or control data to the device. Further, by interleavingamplitude modulated output pulses inbetween the incoming biphasicpulses, output data sensed or generated by the device may be transmittedon the same first and second line conductors used to send an input datastream to the device. Significantly, when the absence of a monophasicpulse is used to signify one binary state, e.g., a binary "0", and anoutput pulse of maximum amplitude is used to signify another binarystate, e.g., a binary "1", a high signal-to-noise ratio may be achieved,allowing the output data to be transmitted over a minimum number ofconductors (two conductors) in a very noisy environment.

It is thus a feature of the present invention to provide a means wherebyimplantable sensors or stimulators may be daisy chained together using aminimum number of connecting conductors.

It is an additional feature of the invention to provide a daisy chain ofimplantable sensor/stimulator devices, serially connected togetherthrough a common power/data bus, wherein each individual device isaddressable from a common controller unit connected by way of the commonpower/data bus to each of the implantable devices.

It is another feature of the invention to provide individual implantablesensors and/or stimulators that can transmit and/or receive power anddata signals over a minimum number of signal lines connectedtherebetween.

It is yet another feature of the invention to provide an implantablesensor/stimulator device having an hermetically sealed part and anon-hermetically sealed part, with electrical feed-through means formaking electrical connections between the hermetically sealed part andthe non-hermetically sealed part, and wherein the hermetically sealedpart encompasses electrical circuits for operating and controlling thedevice, and further wherein the non-hermetically sealed part includes asensor for sensing a condition or substance to which the device isexposed, electrical terminals or pads to which connecting conductors maybe connected, and/or electrodes through which stimulating current pulsesmay be applied to surrounding tissue or body fluids.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 is a block diagram that illustrates multiple sensors/stimulatorsconnected together using a two-conductor bus, which two-conductor busmay be connected to a controller;

FIG. 2 schematically illustrates a preferred manner of how asensor/stimulator made in accordance with the present invention may beconnected with a controller and other sensors/stimulators in a serial ordaisy-chain fashion;

FIG. 3A shows a perspective, partially exploded, view of a preferredsensor/stimulator of the type used in the daisy chain of FIG. 2;

FIG. 3B illustrates a sectional side view of the sensor/stimulator ofFIG. 3A;

FIG. 3C illustrates a sectional top view of the sensor/stimulator ofFIG. 3A;

FIG. 3D illustrates a sectional end view of the sensor/stimulator ofFIG. 3A;

FIG. 4 depicts an implantable lead that includes a plurality of thesensors/stimulators of FIGS. 3A-3D;

FIG. 5A is a functional block diagram of a simple daisy-chainableimplantable device made in accordance with the present invention whereinan electrical path for attaching additional devices passes through anhermetically-sealed portion of the implantable device;

FIG. 5B is a functional block diagram as in FIG. 5A, but wherein theelectrical path for attaching additional devices by-passes thehermetically-sealed portion of the implantable device;

FIG. 5C is a functional block diagram as in FIG. 5A, but whereinadditional circuit functions are provided so that a wide variety ofdifferent sensors and a stimulator may be included within thedaisy-chainable implantable device;

FIG. 6 is a timing diagram that illustrates input and output data sentto and received from a daisy-chainable device of the type shown in FIG.5A, 5B or 5C;

FIG. 7 illustrates a data frame used to communicate with the implantabledevice of the present invention when serially connected in a daisychain;

FIG. 8 is a timing diagram that illustrates time multiplexed input andoutput data within a data frame as it appears on the two-conductor busconnecting a plurality of daisy-chainable devices of the type shown inFIG. 5A, 5B or 5C; and

FIG. 9 shows a representative schematic diagram of a typical lineinterface circuit that may be used as part of the daisy-chainableimplantable devices of the type shown in FIGS. 5A, 5B or 5C.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims.

Turning first to FIG. 1, a block diagram is shown that illustratesmultiple sensors/stimulators 12a, 12b, . . . 12n, or other implantabledevices, connected together, as well as a controller (not shown) usingjust two common conductors 14 and 16. The two conductors 14 and 16provide a common signal and return for data signals and power signalsthat are sent from the controller to the devices 12a, 12b, . . . 12n, aswell as a common signal and return path for data signals transmittedfrom the devices 12a, 12b, . . . 12n, to the controller.

FIG. 2 schematically illustrates how an implantable device, e.g., asensor/stimulator 18a, made in accordance with the present invention maybe connected with a remote controller 20 and other implantable devices18b, . . . 18n, in a serial or daisy-chain fashion. As seen in FIG. 2,the device 18a is connected to the controller 20 by two conductors 14'and 16' which are attached to a first pair of pads or terminals 13 and15 along a proximal side (i.e, the side closest to the controller 20) ofthe device 18a. Another pair of pads or terminals 17 and 19 are locatedalong a distal side (i.e., the side farthest from the controller 20) ofthe device 18a. As will become evident from the description thatfollows, the distal pad 17 is electrically connected to the proximal pad13 through the circuitry 21 located on the device 18a. Similarly, thedistal pad 19 is electrically connected to the proximal pad 15 throughthe circuitry 21 included within the device 18a. Two additionalconductors 14" and 16" are then used to connect the distal pads 17 and19 of the device 18a to corresponding proximal pads 13' and 15' of thenext device 18b connected in the daisy chain. In this manner, as manydevices as desired may be serially connected to the controlled usingjust two conductors.

It is noted that the FIG. 1 is functionally electrically equivalent toFIG. 2. FIG. 2 simply employs proximal and distal pairs of pads orterminals to facilitate the connection of additional devices to thechain by extending two conductors from the distal pads 17 and 19 of amore proximal device in the chain to the proximal pads 13' and 15' of anew device to be added to the chain. However, where the particularapplication allows connections to be made, or branched off of, the twomain conductors 14 and 16, then the configuration of FIG. 1 may be usedjust as well as the configuration of FIG. 2.

There exist many different applications for the daisy-chainablesensor/stimulators 12 or 18 of the present invention illustrated inFIGS. 1 or 2. Generally, where the sensor/stimulators 12 or 18 areimplanted, they are designed to sense one or more body parameters orsubstances found in body tissue or fluids, e.g., glucose level, bloodpH, O₂, temperature, or the like. Such measurements can provide valuableinformation regarding the condition and status of the patient. As such,it is ofttimes desirable to make more than one measurement within thesame general body tissue area so as to be able to compute an average ormean of the measurements thus made, or otherwise obtain a consensus fromseveral different readings, thereby better assuring the accuracy andreliability of the data thus gathered.

Other times, it may be desirable to obtain various measurements of agiven substance at physically-related, but different, body locations.For example, for some applications, e.g., a closed-loop insulin infusionsystem, it could be advantageous to obtain a glucose reading within theblood stream and another glucose reading within the body tissue adjacentthe blood stream. This is because the time constant associated with howrapidly one glucose reading changes compared with the other may bedifferent (and, in fact, is usually different), and being able to obtainor monitor such difference would provide valuable information regardingthe regulation of the insulin infusion.

Turning next to FIGS. 3A, 3B, 3C and 3D, there are shown, respectively,a perspective exploded view (FIG. 3A), a side view (FIG. 3B), a top view(FIG. 3C), and an end view (FIG. 3D), of a typical implantable sensordevice 30 of a type suited for use with the present invention. As seenbest in FIG. 3A, the sensor device 30 typically includes a carrier orsubstrate 36 on which an integrated circuit (IC) 38 and othercomponents, such as a capacitor 40, are mounted. In some embodiments, itshould be noted that the carrier or substrate 36 may actually comprisethe substrate on which the IC 38 is fabricated; but for purposes of theexplanation which follows, it is assumed that a separate substrate orcarrier 36 is employed with various circuit elements mounted thereon toform a hybrid circuit. The carrier or substrate has conductive patternsetched or otherwise deposited thereon to interconnect the IC 30,capacitor 40, and any other components to form a hybrid circuit whichcarries out the desired sensing (or other) function.

All of the components of the hybrid circuit are hermetically sealedwithin a cavity formed by a lid or cover 42 which is bonded to thesubstrate 36. Proximal pads or terminals 13 and 15, as well as distalpads or terminals 17 and 19, remain outside of the hermetically sealedpart of the hybrid circuit created by the cover 42. These proximal anddistal pads, however, are electrically connected to the circuitry withinthe hermetically sealed part through suitable feedthrough connections.While any suitable feedthrough connection may be used for this purpose,a preferred manner of making such feedthrough connection is to use afeedthru connection that passes through the carrier or substrate in thestair-step manner (including both vertical and horizontal segments)disclosed in U.S. patent application Ser. No. 08/515,559, filed Aug. 16,1995, now U.S. Pat. No. 5,750,926, entitled "Hermetically-SealedElectrical Feedthrough For Use With Implantable Electronic Devices",which application is assigned to the same assignee as is the instantapplication, and which application is incorporated herein by reference.

On the side of the carrier or substrate opposite the hybrid electricalcircuitry, a suitable electrochemical sensor 44, or other desired typeof sensor or stimulator, may be formed or located. A type ofelectrochemical sensor that may be used, for example, is the enzymeelectrode sensor described in U.S. Pat. No. 5,497,772, incorporatedherein by reference, and in particular, in FIGS. 2A, 2B, 2C, 3, 4A and4B of that patent. However, it is to be emphasized that the precisenature of the sensor 44, or other implantable element used within thedevice 30, is not critical to the present invention. All that matters isthat the sensor or other element be implantable, and that it provide adesired function, e.g., sense a certain type of parameter of substance,or generate a certain type of signal, in response to an appropriatecontrol signal or signals.

Whatever, type of control signal(s) or output signal(s) is/are generatedby the sensor 44, or other element, such signal(s) may be communicatedfrom the hybrid circuit side of the substrate or carrier 36 (which isthe top side as the device 30 is oriented in FIG. 3B or FIG. 3D, andwhich top side includes the hermetically sealed portion of the device)to the sensor side of the device 30 (which is the bottom side as shownin FIG. 3B or 3D) by way of appropriate hermetically-sealed feedthroughsthat pass step-wise from the hybrid (top) side of the device 30 throughthe substrate or carrier, e.g., in the manner set forth in theabove-referenced '559 patent application, to the sensor (bottom) side ofthe device 30.

For example, where the sensor comprises a glucose sensor of the typetaught in U.S. Pat. No. 5,497,772, there may be five conductors thatelectrically interface with the main elements (electrodes) of thesensor, as seen best in FIG. 4A of the '772 patent. Where such a glucosesensor is employed, these five conductors thus interface with the hybridelectrical circuitry found on the top side of the carrier 36) usingappropriate feedthroughs that hermetically pass step-wise through thecarrier 36, i.e., that pass through the carrier using both vertical andhorizontal segments, as taught in the '559 application.

It is to be emphasized that the invention is not limited to the specificsensor configuration shown in FIGS. 3A-3D. Rather, any type ofimplantable device, whether configured as illustrated in FIGS. 3A-3D orotherwise, could be used with the invention. The present inventionrelates to the manner in which multiple sensors, or other implantabledevices, regardless of their shape or configuration, may be seriallyconnected in daisy-chain fashion using a minimum number of conductors,e.g., two conductors, as well as to the manner in which such deviceselectrically communicate with the controller 20 or other remote deviceso that each is individually addressable by the controller, and so thateach can send data to the controller. Other aspects and features of suchinvention, e.g., the manner in which some of the circuitry containedwithin the hermetically-sealed portion of the sensor device 30 operateso as to consume very low power, are the subject of separateapplications filed concurrently herewith. Such concurrently-filed andpending applications, assigned to the same assignee as the presentapplication, include: (1) A LOW POWER CURRENT-TO-FREQUENCY CONVERTER FORUSE IN IMPLANTABLE SENSORS (Attorney Docket No. 57794), Ser. No.08/928,868, filed Sep. 12, 1997; and (2) A LOW POWER RECTIFIER CIRCUIT(Attorney Docket No. 57795), Ser. No. 08/928,871, filed Sep. 12, 1997;which applications are expressly incorporated herein by reference.

It is noted that the configuration of FIG. 2 is especially well-suitedwhere several of the implantable devices are to be daisy-chainedtogether to form a single lead 32, as shown in FIG. 4. As seen in FIG.4, three sensor-type devices 30a, 30b, and 30c are connected togethervia lead segments 46a, 46b, and 46c. Each of the lead segments 46a, 46b,and 46c, contain two conductors 14, 16, and may be constructed in anysuitable manner, e.g., with the two conductors being spirally woundwithin the lead segments, and with the spiral windings being encased orcovered within a sheath of silicone rubber, as is known in the lead art.(Note, that for purposes of FIG. 4 each of the two conductors 14, 16within the lead 32 is considered as one conductor, even though each issegmented within the individual lead segments 46a, 46b and 46c as itconnects from the distal pad of one device to the proximal pad ofanother device.) A distal cap 34 covers the distal pads of the end, ormost-distal, device 30c of the lead 32.

As was mentioned above, it is noted that the device 30 need notnecessarily employ a carrier 36, per se, as shown in FIGS. 3A, 3B, 3C,3D and FIG. 4, wherein the control electronics are positioned on oneside (the top side) of the carrier 36, and the sensor, or other devicebeing used with or controlled by the electronics is placed on the otherside (the bottom side) of the carrier. Rather, a ceramic or othersubstrate on which the IC 38 is formed may itself function as thecarrier. That is, the vias that are formed in a substrate, or betweenvarious layers of an integrated circuit as the integrated circuit (IC)is formed, may function as hermetic feedthroughs, with selected layersand traces being coated as needed with aluminum oxide, or other oxidecoatings, in the manner taught in the aforementioned '559 patentapplication, and/or in U.S. provisional application Ser. No. 60/033,637,filed Dec. 20, 1996 (Attorney Docket No. 57720), incorporated herein byreference, in order to seal appropriate sections or portions of the ICso that the coated IC may itself be implanted. In such embodiment, thesensor or other implantable element 44 used with or controlled by the ICmay be formed on the back side of the IC's substrate. Thus, a carrier,per se, is not needed because the IC substrate functions as the carrier.

An important feature of the present invention is the electricalcircuitry contained within or included as a part of what is referred toabove as the "hybrid circuit portion" of the implantable device 30. Thepurpose of this electrical circuitry is to allow the implantable device30 to be daisy chained with other similar implantable devices, whilestill allowing each individual device to be individually addressed,controlled and monitored from a single controller 20. This electricalcircuitry, frequently referred to hereafter as the interface/controlcircuitry, is shown in FIGS. 3A, 3B, 3C, 3D and 4 as being located onthe "top" side of the carrier 36, predominantly underneath the cover 42in an hermetically sealed portion of the device 30. It is to beunderstood, however, that the location of the interface/controlcircuitry within the device 30 is not critical so long as it isappropriately hermetically sealed.

The control/interface circuitry may take many and varied forms. FIGS.5A, 5B and 5C, discussed below, show three such variations. Turningfirst to FIG. 5A, for example, a functional block diagram of a basicconfiguration of control/interface circuitry 50 for use with a singlesensor 52 made in accordance with the present invention is shown. Thedotted line 54 represents an hermetic seal that hermetically seals thecircuitry 50 and all but a portion of the sensor 52. (Generally, where asensor is employed, at least a portion of the sensor, e.g., anelectrode, is left exposed to the tissue and fluids within which thedevice is implanted so that the sensor can perform its intended functionof sensing some parameter or element present within the tissue and/orfluids.) The input pads 13 and 15, as well as the output pads 17 and 19,are not hermetically sealed, thereby allowing these pads to be readilyconnected to the two conductors 14 and 16 (FIG. 1) from the controller20.

As seen in FIG. 5A, pads 13 and 15 are connected to respectiveconductive traces, labeled LINE 1 (IN) and LINE 2 (IN), and each ofthese conductive traces passes through respective feedthroughs 53 and 55into the hermetically sealed portion of the circuitry 50. Pads 17 and19, on the other side of the circuit, are likewise connected torespective conductive traces, labeled LINE 1 (OUT) and LINE 2 (OUT), andeach of these conductive traces passes through respective feedthroughs57 and 59 into the hermetically sealed portion 54 of the circuitry 50.Inside the hermetically sealed portion, LINE 1 (IN) connects with LINE 1(OUT) via conductive trace 56, and LINE 2 (IN) connects with LINE 2(OUT) via conductive trace 58. In this manner, pad 13 is electricallyconnected with pad 17 via trace 56 which passes through the hermeticallysealed portion 54 between feedthroughs 53 and 57. This interconnectionof pad 13, trace 56 and pad 57 may be referred to simply as LINE 1.similarly, pad 15 is electrically connected with pad 19 via trace 58,which trace also passes through the hermetically sealed portion 54between feedthroughs 55 and 59. This interconnection may be referred tosimply as LINE 2.

As seen in FIG. 5A, a power rectifier circuit 60 is connected betweenLINE 1 and LINE 2. This circuit extracts and rectifies any signal pulsesfound on LINE 1 and LINE 2 and produces an operating voltage, +V and -V,for powering the circuitry 50. Such rectification is not a trivial taskgiven the low level signals which are generally present on LINE 1 andLINE 2, and given the fact that initially (in the absence of a signal tobe rectified), there is no power yet available with which to operate therectifier circuit. Details of such circuitry may be found in applicant'scopending patent application Ser. No. 08/928,871, filed Sep. 12, 1997, ALOW POWER RECTIFIER CIRCUIT (Attorney Docket No. 57795), previouslyincorporated herein by reference.

A line interface circuit 62 also is connected between LINE 1 and LINE 2.The circuit 62 functions as an interface between the circuitry 50 andLINE 1 and LINE 2. To this end, the interface circuit 50 receivesincoming data pulses present on LINE 1/LINE 2 and generates a DATA-INsignal on line 64 therefrom. The interface circuit 62 further generatesa clock (CLK) signal on line 66 that is synchronized with the incomingdata signals. The interface circuit 50 also receives digital outputdata, DATA OUT, from a counter circuit 68, and converts this output datato an appropriate format prior to placing the output data back on LINE1/LINE 2. One type of line interface circuit 62 that may be used withthe circuitry 50 is illustrated in the schematic diagram shown andexplained below in conjunction with FIG. 9.

Still referring to FIG. 5A, the sensor 52 may be any suitableimplantable sensor adapted to sense a desired condition, parameter, orsubstance present (or absent) in the implantable tissue within which thedevice 30 is implanted. For example, the sensor 52 may comprise aglucose sensor that generates an output analog current, I, appearing online 69, having a magnitude that varies as a function of the sensedglucose.

As a practical matter, regardless of the type of sensor 52 that isemployed, it will usually generate either an analog output voltage or ananalog output current as a function of the concentration, magnitude,composition, or other attribute, of the parameter being sensed. Suchanalog current or voltage may then be converted, using an appropriateconverter circuit, to a form that is more suitable for transmission.

While many different types of converter circuits may be employed, e.g.,analog-to-digital (A/D) circuits as are known in the art, a preferredtype of converter circuit is a voltage-to-frequency (V/F) converter or acurrent-to-frequency (I/F) converter, which converts a low level inputsignal (voltage or current) to a frequency signal. Such frequency signalappears on line 72. Typically, the frequency signal on line 72 comprisesa train of pulses having a frequency (or repetition rate) that varies asa function of the input voltage or current. In FIG. 5A, for example, itis assumed that the sensor 52 generates an output current I, and thatthe converter circuit 70 comprises a current-to-frequency (I/F)converter circuit, generating an output pulse train on line 72 that hasa frequency which varies as the magnitude of the current I varies. (Ofcourse, it would be just as feasible for the sensor to generate anoutput voltage V, and have the converter circuit 70 comprise avoltage-to-frequency (V/F) converter circuit, generating an output pulsetrain on line 72 having a frequency that varies as the magnitude of thevoltage V varies.)

Once a pulse train 72, or other ac signal, is generated having afrequency which varies as a function of the parameter being sensed bythe sensor 52, such signal is applied to a counter circuit 68. (Note, asa shorthand notation used in this application, a signal appearing onsignal line having a given reference number may be referred to as thesignal having such given reference number, i.e., the signal appearing onsignal line 72 may simply be referred to as "signal 72".) The countercircuit simply counts the number of pulses present in the signal 72 overa prescribed period of time, thereby providing a measure of thefrequency of the signal 72. For example, if the signal 72 comprises asignal having 100 pulses per second (pps), and if the counter 68 is setto count the pulses over a period of time, or time-measurement window,of one second, then the counter 68, assuming it is reset to zero at thebeginning of the measurement period, will have a count of 100 storedtherein at the end of the measurement period. If the frequency of thesignal 72 increases, e.g., to 120 pps, then the count held in thecounter should increase to 120 at the end of the measurement period. Ifthe frequency of the signal 72 decreases, e.g., to 80 pps, then thecount held in the counter should decrease to 80 at the end of themeasurement period. In this manner, by resetting the counter 68 at thebeginning of each measurement period, the count held in the counter atthe end of the measurement period provides a signal representative ofthe frequency of the signal 72. Such count signal, for the basicembodiment shown in FIG. 5A, may thus serve as the output data signal,DATA OUT, that is sent to the line interface circuit 62 over signal line74.

Control of the counter 68, i.e., resetting the counter and/or stoppingthe counter after a prescribed measurement period, is controlled bycontrol logic 76. In a simple embodiment, the measurement period may bea fixed time period. In other embodiments, the measurement period may beset by input data received over signal line 64 from the line interfacecircuit 62. The clock signal 66 may be used as a measure of elapsedtime, as well as to coordinate when the counter 68 sends its DATA OUTsignal 74 to the line interface circuit 62.

As needed, a voltage generator circuit 78 generates a reference voltageV_(REF), and a bias signal, V_(BIAS), that are used by thecurrent-to-frequency (I/F) converter circuit 70 as it performs itsfunction of converting the analog current signal 69 to a frequencysignal 72, as explained more fully in the above-referenced copendingpatent application Ser. No. 08/928,868 A LOW POWER CURRENT-TO-FREQUENCYCONVERTER FOR USE IN IMPLANTABLE SENSORS, Attorney Docket No. 57794,previously incorporated herein by reference.

Turning next to FIG. 5B, there is shown a functional block diagram of analternative implantable device 50'. The device 50' is in most respectsthe same as, or similar to, the device 50 shown in FIG. 5A. That is, thedevice includes an hermetically-sealed portion 54' wherein desiredelectronic circuitry is housed, including a power rectifier circuit 60'and a line interface circuit 62'. Other circuits, representedgenerically as the block 80, are coupled to the line interface circuit62'. Such other circuits 80 may include, e.g., an I/F converter or othertype of converter, a sensor, a stimulator, a counter, a microprocessor,and/or other circuits as needed to control and perform a desiredstimulation or sensing function.

Like the device 50 of FIG. 5A, the device 50' of FIG. 5B includes a pairof feed through terminals 53' and 55' through which a LINE 1 and a LINE2 connection may be respectively made between external (non-hermeticallysealed) input terminal pads 13' and 15' and the hermetically sealedpower rectifier circuit 60' and line interface circuit 62'. Unlike thedevice 50 of FIG. 5A, the device 50' of FIG. 5B includes output terminalpads 17' and 19' for LINE 1 and LINE 2 that are connected directly tothe input terminal pads 13' and 15', respectively, without theconnection passing through the hermetically sealed portion 54' of thedevice. As such, the configuration of the device 50' illustrated in FIG.5B is better suited for applications where the sensor/stimulator devicesin the daisy chain do not have to be lined up in serial or lead-likefashion (as suggested, e.g., by the devices 18a, 18b . . . 18n in FIG.3), but wherein each device 50' of the chain may be fanned out andlocated or positioned at varying locations relative to the other devicesin the chain.

Next, with reference to FIG. 5C, there is shown a functional blockdiagram of a sensor stimulator device 50". The device 50" is, in manyrespects, similar to the device 50 shown in FIG. 5A, but as is evidentfrom FIG. 5C, the device 50" includes additional circuit functions thatallow a wide variety of different sensor and/or stimulator functions tobe provided. With the exception of stimulating electrodes 82 and 83, andportions of one or more of the sensors 53a, 53b, 53c, . . . 53n, all ofthe elements shown in FIG. 5C are included in an hermetically-sealedportion of the device 50".

Like the device 50 of FIG. 5A, the device 50" of FIG. 5C includes apower rectifier circuit 60" and a line interface circuit 62", each ofwhich is connected to input/output LINE 1 and LINE 2 conductors. TheseLINE 1 and LINE 2 conductors are connected through appropriatefeed-through elements (not shown) to appropriate pads (also not shown inFIG. 5C, but which are the same as or similar to the input/output pads13, 15, 17 and 19 of FIG. 5A, or input/output pads 13', 15', 17'or 19'shown in FIG. 5B). The power rectifier circuit 60" and the lineinterface circuit 62" serve the same function as, and may be the same asor similar to, the power rectifier circuit 60 and line interface circuit62 described or referenced above in connection with FIG. 5A. Also, likeor similar to the device 50 of FIG. 5A, the device 50" shown in FIG. 5Cmay include a voltage generator circuit 78" that generates a V_(REF) andV_(BIAS) signal used by various other circuits within the device 50".

Unlike the device 50 of FIG. 5A, which includes a single sensor 52, thedevice 50" of FIG. 5C includes multiple sensors 53a, 53b, 53c, 53d, . .. 53n. Each of these sensors may be configured to sense a differentparameter or substance, or all may be configured to sense the sameparameter or substance. Alternatively, a first group of the sensors,e.g., sensors 53a and 53b, may be configured to sense the same parameteror substance; while a second group of the senors, e.g., sensors 53c,53d, . . . 53n, may be configured to sense a different parameter orsubstance; while yet a third sensor or group of sensors, e.g., a sensorwhich generates a voltage V_(C), may be configured to sense yet anadditional parameter or substance. For example, sensors 53a and 53b maycomprise strain gauges which measure movement of the tissue in whichthey are implanted; sensors 53c, 53d, . . . 53n may comprise glucosesensors that sense the glucose concentration of the tissue or fluids inwhich they are implanted, and the voltage V_(C) may represent a voltageobtained from within the device 50" that represents a measure of thetemperature of the device 50", and hence a measure (over time) of thetemperature of the tissue in which the device 50" is implanted.

As shown in FIG. 5C, the sensors 53a and 53b each generate, as an outputsignal (having a magnitude that varies in some known fashion, e.g.,linear, as a function of the parameter or substance being sensed) anoutput current I_(a) and I_(b), respectively. As required, the voltageV_(C), which may represent a status condition or other parameter relatedto the device 50", may be applied to a voltage-to-current convertercircuit 88, which circuit converts the sensed voltage V_(C) to acorresponding current, I_(c). The currents I_(a), I_(b), and I_(c) areall connected through an analog multiplexer circuit 90 to a firstcurrent-to-frequency (I/F₀) converter circuit 71a. The multiplexer 90 iscontrolled by a suitable control signal received over signal line 92from state machine control logic 94.

The I/F₀ converter circuit 71a produces a variable frequency outputsignal (having a frequency that varies as a function of the selectedinput current I_(a), I_(b), or I_(c)) which is directed to a firstmeasurement counter 98a through a digital multiplexer 96. As shown inFIG. 5C, the digital multiplexer 96 may also select, under control of acontrol signal 95 received from the state machine control logic 94, theoutput frequency signal generated by a second current-to-frequency(I/F₁) converter circuit 71b, which I/F₁ converter circuit 71b receivesan input current I₁ from sensor 53c. Thus, the first measurement counter98a measures either the frequency of the output from the I/F₀ circuit71a or the frequency of the output from the I/F₁ circuit 71b ascontrolled by the state machine control logic 94.

Other measurement counters 98b, . . . 98n may also be used torespectively measure the frequency signals generated by additionalsensors 53d, . . . 53n. That is, sensor 53d generates an output currentI₂ as a function of a sensed parameter or substance. This current 12 isapplied to a third current-to-frequency (I/F₂) converter circuit 71c.The I/F₂ circuit 71c converts the current I₂ to a frequency signal,which frequency signal is applied to the measurement counter circuit98b. In a similar manner, each other sensor that may be present in thedevice 50", up to sensor 53n, generates a corresponding output currentas a function of a sensed parameter or substance. The current from eachsuch sensor, including current I_(n) from sensor 53n, is applied to acorresponding current-to-frequency converter circuit, up to an includingan I/F_(n) converter circuit 71n. Each such I/F converter circuit, up tothe I/F_(n) converter circuit 71n, converts its respective input currentto a corresponding frequency signal, which frequency signal is thenapplied to a respective measurement counter circuit. Thus, for example,sensor 53n generates current I_(n) as a function of a sensed parameteror substance, and applies this current I_(n) to converter I/F_(n), thefrequency output signal of which is then applied to measurement counter98n.

The output signals from each of the measurement counters 98a, 98b, . . .98n, which output signals represent a digital measure of the parametersensed by a corresponding sensor that is upstream from the measurementcounter, are then selectively applied, through an output multiplexercircuit 100 to the line interface circuit 62". This selection iscontrolled by the state machine control logic 94 by way of a suitablecontrol signal 97. The output signal selected by the multiplexer 100thus comprises output data (referred to in FIG. 5C as "DATA OUT") thatis applied to the LINE 1 and LINE 2 conductors of the two-conductorcable (or connecting bus) that connects each of the implantable devices50" to a suitable controller 20. Thus, this output data may betransferred to the controller 20 over the LINE 1/LINE 2 conductors inthe manner described below.

As described above, it is thus seen that the device 50" allows multiplesensors 53a, 53b, . . . 53n, to sense an appropriate parameter,substance, or condition, convert such sensed parameter, substance orcondition to first a frequency signal, and second to a digital signal,which digital signal may then be selectively passed on to other devices,e.g., a remote controller 20, or another device 50, coupled to the LINE1/LINE 2 conductors. Advantageously, through the use of the multiplexers90, 96 and 100, each of which is controlled by suitable state machinecontrol logic 94, the frequency at which a given sensor is sampled canbe controlled in a desired manner. For example, as suggested by theconfiguration shown in FIG. 5C, it is seen that sensors 53d . . . 53nmay be sampled at a rate controlled by output multiplexer 100. Sensor53c may similarly be sampled at a rate controlled by output multiplexer100 and multiplexer 96; and sensors 53a and 53b, as well as statusvoltage V_(c), may be sampled at a rate controlled by output multiplexer100, multiplexer 96 and multiplexer 90.

It is to be emphasized that the particular configuration (shown in FIG.5C) of using multiple sensors and multiplexers is only exemplary, notlimiting. It is intended that any configuration of an implantable,daisy-chainable device that allows one or more, e.g., multiple, sensorsto be employed within the device, with the data sensed by each sensorbeing convertible to an appropriate form and transferrable back througha suitable line interface circuit to the LINE 1/LINE 2 conductors, beingincluded be included within the scope of the invention.

Still referring to FIG. 5C, it is seen that the device 50" furtherincludes a stimulator circuit 86. The stimulator circuit 86 iscontrolled by the state machine control logic 94. The stimulator circuit86 generates appropriate electrical stimulation pulses, applied acrossone or more electrodes 82 and/or 83. These electrodes 82 and/or 83 areconnected to the stimulator circuit 86 through conductors that passthrough an appropriate feed-through terminal, or seal 84, therebyallowing at least one of the electrodes to be in the non-hermeticallysealed portion of the device 50". The design of the stimulator circuit86 may be conventional, e.g., as is commonly used in the cochlearstimulation art, see. e.g. U.S. Pat. No. 5,603,726 (incorporated hereinby reference), the cardiac tissue stimulation art, the neuralstimulation art, and/or the pain-relief stimulation art.

The state machine control logic 94 may likewise be of conventionaldesign, appropriate for the low power constraints applied to all of thecircuitry within the device 50". It is the function of the state machinecontrol logic 94 to generate control signals that control the operationof the stimulator circuit 86, as well as the sampling and dataconversion associated with each of the various sensors 53a, 53b, . . .53n, all as controlled by and in synchrony with DATA-IN signals and aclock (CLK) signal received from the line interface circuit 62". In itssimplest form, the control logic 94 may comprise simply a flip-flop ortwo, and associated logic gates, which receive the DATA-IN and CLKsignals and use such signals to steer or control the flip flop(s) totoggle between, or cycle through, various operating states. In eachstate, appropriate control signals are generated by the control logic tocause a specified sensor to be sampled, and/or a stimulation pulse to begenerated. As required, reset signals are also generated in the variousstates in order to reset the various counter circuits. Additionally, apower-on-reset circuit 102 may be coupled to the state machine controllogic 94 to assure that the control logic 94 comes up in a desiredstate, or operating mode, each time power is applied to the device 50".The details of the state machine control logic are not critical forpurposes of the present invention. Those of skill in the art can readilyfashion appropriate state machine control logic to accomplish a desiredoperating performance for the device 50".

It should also be pointed out that any circuitry which accomplishes thefunction of the state machine control logic 94 could be used in lieu of,or as a supplement to, the conventional state machine control logicdescribed above. Such circuitry includes, for example, a low powermicroprocessor programmed with an operating program stored in read-onlymemory (ROM).

Further, it is noted that inclusion of a stimulator circuit 86, andcorresponding stimulation electrodes 82 and/or 83, within theimplantable device 50" should be viewed as an option, not a requirement.

That is, for many applications, all that the daisy-chainable implantabledevice 50" need do is to sense one or more parameters or substances,without the need to provide stimulation. For such applications, thestimulation function may thus be omitted.

The current-to-frequency converter circuits 71a, 71b, . . . 71n may eachbe substantially as is described in the previously-referenced patentapplication Ser. No. 08/928,868 (incorporated herein by reference)entitled LOW POWER CURRENT-TO-FREQUENCY CONVERTER CIRCUIT FIT USE INIMPLANTABLE SENSORS.

The counter circuits 98a, 98b, . . . 98n, as well as the multiplexercircuits 90, 96 and 100, may all be of conventional design, utilizing,e.g., low power CMOS integrated circuits.

Turning back momentarily to FIG. 2, where a plurality of implantable,daisy-chainable sensor/stimulator devices 18a, 18b . . . 18n made inaccordance with the present invention are shown connected in tandem, itis noted that a key feature of the present invention is the ability ofthe controller 20 to provide operating power to, and to individuallysend control data to, as well as receive data from, each of the devices18 that are connected to the controller over the same two conductors 14and 16. The preferred manner in which such powering and individualaddressing is done is next explained in connection with FIGS. 6, 7 and8.

FIG. 6 illustrates a timing diagram that shows the preferredrelationship between input data (top waveform) sent to the implantabledevices and output data (bottom waveform) received from the implantabledevices, as such data would appear on the two LINE 1/LINE 2 conductorsthat connect all of the devices together. In FIG. 6, it is assumed thattime is the horizontal axis, whereas amplitude is the vertical axis. Itis also noted that the waveforms shown in FIG. 6 represent currentwaveform pulses.

As seen in FIG. 6, the preferred form for the input data is biphasicpulses. Each biphasic pulse comprises a first current pulse of a firstpolarity, followed by a second current pulse of the same magnitude ofthe opposite polarity. Thus, the net current for each biphasic pulse ispreferably zero, with the positive current pulse effectively balancingout the negative current pulse. The typical widths of the current pulsesare from 1 to 100 microseconds (μsec), with the magnitude of eachcurrent pulse typically ranging from 10 to 1000 microamps (μamp). Abinary "1" may be represented by a biphasic pulse of one phase, e.g., apositive current pulse followed by a negative current pulse; while abinary "0" may be represented by a biphasic pulse of the opposite phase,e.g., a negative pulse followed by a positive pulse. Thus, as shown inFIG. 6, a binary "1" may be represented as a positive current pulsefollowed by a negative current pulse, while a binary "0" is representedby a negative current pulse followed by a positive current pulse.

As further seen in FIG. 6, there may be an "off time", e.g., of 0 (zero)to 10 μsec, between the two current pulses of each biphasic pulse, butsuch off time is not always necessary (hence, as indicated, it may bereduced to zero). A prescribed time increment T1 separates one inputdata biphasic current pulse from the next input data biphasic currentpulse.

As also seen in FIG. 6, the preferred form for the output data is also abiphasic pulse, amplitude modulated (or preferably ON/OFF modulated) asa function of whether the output data is a binary "1" or "0". In thepreferred embodiment, the peak amplitude of the output data pulse for abinary "1" is I_(P), while the peak amplitude of the output data pulsefor a binary "0" is zero. Thus, in this preferred ON/OFF modulationscheme, the presence of an output data pulse represents a binary "1" andthe absence of an output data pulse represents a binary "0". Output datapulses are inserted in the data stream appearing on the LINE 1/LINE 2conductors pulses at a specified time T2 from the input data pulse so asto fall between the input data pulses, in a time-division multiplexedmanner. Although the preferred form of the output data pulses is abiphasic pulse (to achieve current balancing), it is noted that in someinstances a monophasic pulse at time T2 (and with amplitude of I_(P) orzero) may be used.

As shown in FIGS. 7 and 8, the input data sent over the LINE 1/LINE 2conductors by the controller is divided into data frames of length T3.Note that the data frame shown in FIG. 7 represents a different dataframe than that which is shown in FIG. 8. Within each data frame, N bitsof data are found, where N is an integer typically ranging from 4 to 64.FIG. 7 illustrates a generalized data frame of N bits; while FIG. 8illustrates a data frame of six bits.

A representative assignment of the data bits included in a generalizeddata frame is illustrated in FIG. 7. As shown in FIG. 7, the first bitof the data frame is a start bit, followed by three bits (bits 2, 3 and4) that comprise address bits. Bits 5, 6 and 7 comprise an operationcode (op code) that define an operation (e.g., one of eight operations)that the addressed device is to perform. Bits 8-12 then defineparticular control parameters associated with the operation that is tobe carried out, e.g., the amplitude of a stimulation pulse, or theparticular sensor(s) from which data is to be gathered, etc. Bit 13comprises a parity bit. Bits 14 through N-1 comprise transmit data,followed by the Nth bit, which is a stop bit or end bit of the dataframe.

The transmit data bits, in one embodiment, effectively define timewindows within the data frame during which data being sent (ortransmitted) from a given implantable device to the controller is to beinserted in the data frame. For example, the controller may be set up torecognize that output data appearing T2 seconds after the 13th inputdata pulse (i.e., during Bit 13) corresponds to data sent by a specificone of the devices in the daisy chain. In this manner, some data fromeach of the implantable devices in the chain may be received during eachdata frame. Alternatively, other schemes may be used to keep track ofwhich output data appearing on the LINE 1/LINE 2 conductors belongs towhich devices. For example, all of the devices but one may beeffectively shut down insofar as sending output data is concerned untilall the needed data from that one enabled device has been obtained. Thisallows data to be received much faster from a given device, but at theexpense of not receiving data from the other devices until thetransmission is complete.

Because the input data comprises biphasic pulses that occur at a regularinterval or rate (e.g., every T1 seconds), the energy contained in suchpulses may be utilized to provide the operating power for the circuitscontained within the device 50". Such is accomplished using therectifier circuit 60, 60' or 60" (FIGS. 5A, 5B or 5C). A preferredrectifier circuit is described in the above-referenced co-pending patentapplication Ser. No. 08/928,871 (incorporated herein by reference)entitled LOW POWER RECTIFIER CIRCUIT.

The input and output data pulses of the type shown in FIGS. 6 and 8 aregenerated by the line interface circuit 62, 62' or 62" (FIGS. 5A, 5B or5C). A schematic diagram of a representative line interface circuit isshown in FIG. 9. The transistor elements included in the circuit of FIG.9 are of the P-MOS FET or N-MOS FET type. Representative sizes of suchtransistor elements are listed in Table 1. The N-MOS FET and P-MOS FETdimensions shown in Table 1 relate to the relative size of eachtransistor as it is formed on the IC substrate. More particularly, anN-MOS FET, for example, having a size of "5/10" means that the width ofthe source to drain channel is 5 microns (where "micron" means onemicrometer, also written as 1 μm, or 1×10⁻⁶ meters), and the length ofthe channel is 10 microns. This type of characterization (by dimensionor size) of the various N-MOS FET and P-MOS FET transistors used withinan IC is known and understood by those of skill in the semiconductorprocessing art. Advantageously, by selectively controlling the size(dimensions) of such transistors during the IC processing steps, theperformance of the N-MOS and P-MOS FET transistors can be controlled ortailored for the specific design for which the transistor is used. Thusa relatively "long" N-MOS FET, having a size of, e.g., 5/10, may exhibita higher turn-on resistance (and hence a slower turn on time) thanwould, e.g., a relatively "short" N-MOS FET, having a size of 4/4.

                  TABLE 1                                                         ______________________________________                                        Representative Sizes of Transistor Elements of FIG. 9                         Device        Type    Size (W/L μm)                                        ______________________________________                                        M1            P-MOS   100/0.8                                                 M2            N-MOS   50/0.8                                                  M3            N-MOS   10/0.8                                                  M4            N-MOS   10/0.8                                                  M5            P-MOS   2.4/0.8                                                 M6            N-MOS   2.4/200                                                 M7            P-MOS    4/0.8                                                  M8            P-MOS    4/0.8                                                  M9            P-MOS    4/0.8                                                   M10          N-MOS   2.4/5.4                                                 ______________________________________                                    

The particular line interface circuit shown in FIG. 9 receives biphasicpulses from and sends monophasic pulses back to the controller 20 (FIG.2). The operation of the circuit shown in FIG. 9 should be self-evidentto those of skill in the art. The following explanation provides anoverview of such operation.

The biphasic pulses appearing on the LINE 1/LINE 2 conductors areeffectively pulse pairs, one pulse of the pair being positive and theother being negative, with the polarity of the first pulse determiningwhether the pulse represents a binary "1" or a binary "0". The LINE1/LINE 2 conductors are connected to CMOS switches M3 and M4 such whenLINE 1 goes high, it switches on M4 and switches off M3. Switching on M4pulls down the level at the drain of M4 for the duration of the pulse.Switching M3 off makes the drain of M3 go high. The low pulse at thedrain of M4 passes through three inverters 112, 114 and 116 and appearsas a high data pulse at the DATA-IN signal line 64, having the samepulse width as the incoming pulse. Thus, incoming positive pulses onLINE 1 are passed through as positive pulses on the DATA-IN line 64.

In a similar fashion, negative pulses on LINE 1 cause M3 to switch ONand M4 to switch off. With M4 switched off, the drain of M4 is pulledhigh, and this high level passes through inverters 112, 114 and 116 tobecome a low level data pulse at the DATA-IN signal line 64 having thesame pulse width as the incoming negative pulse. Thus, incoming negativepulses on LINE 1 are passed through as low pulses on the DATA-IN line64.

The clock signal, on signal line 66, is generated by a D-flip-flop 122,as controlled (or clocked by) the rising edge of the Q* output of a NORlatch 120.

The NOR latch 120 is reset by gate 118, causing the Q* output of latch120 to rise, whenever either one of the M3 or M4 switches are turned on,which occurs whenever an input pulse of either polarity is received.Having the NOR latch 120 reset, thereby causing its Q* output to risefrom a low level to a high level, causes two events to happen: (1) afirst timer circuit 124 starts its timing cycle; and (2) the D flip-flop122 is clocked, causing its Q output (attached to signal line 66) to beset to a high level. The rising edge of the signal on signal line 66further causes a second timer circuit 126 to be triggered.Representative times for the first and second timers are 152microseconds (μsec) and 44 μsec, respectively. Thus, as soon as an inputpulse is received, positive or negative, both the first and secondtimers start to time out.

For so long as the first timer circuit 124 is timing out, e.g., for 152μsec following an input pulse, it locks the NOR latch 120, making itimmune to any further signals received from the M3 or M4 switches (i.e.,making it immune to any line activity appearing on the LINE 1/LINE 2signal lines). Line activity during this immune time typically includesthe second line pulse of the biphasic pulse pair, any reply pulse thatmay be deliberately placed on the LINE 1/LINE 2 signal lines (asexplained below), or noise. As soon as the first timer 124 times out,the NOR latch 120 is unlocked, making it responsive to the next lineactivity that occurs, e.g., the next biphasic data pulse, representingthe next data bit of the data frame.

When the second timer 126 times out, e.g., at 44 μsec after receipt ofan incoming pulse, the Q output of the second timer 126 goes high,causing the D flip-flop 122 to reset, thereby causing the clock signalon signal line 66 go low. The clock signal on signal line 66 remains lowuntil the D flip-flop 122 is again clocked high by the NOR latch 120upon receipt of the next input pulse over the LINE 1/LINE 2 conductors(after expiration of the 152 μsec immune period set by the first timer124). Thus, the clock signal CLK (on signal line 66) comprises a signalthat goes high upon receipt of the first pulse over the LINE 1/LINE 2conductors, remains high for the second timer period, e.g., 44 μsec, andthen goes low and remains low until receipt of the next input pulse overthe LINE 1/LINE 2 conductors after expiration of the first timer period(e.g., 152 μsec). For this reason, the data transmission cycle from theremote controller 20 (FIG. 2) should not be shorter than the longestpossible time of the first timer 124, taking into account variations inthe timer period as a result of process variations. For example,assuming a nominal first timer value of 154 μsec, a safe value for thedata transmission cycle is about 244 μsec, which allows for about a ±52%variation in the first timer period.

Note that there is a shorter propagation delay from receipt of a pulseon the LINE 1/LINE 2 input lines to the DATA-IN line 64 than to the CLKline 66. This is important in forming a buffered data line 127 usinganother D flip-flop 128. Such a buffered data line 127 is created byconnecting the DATA-IN line 64 to the D-input of the D flip-flop 128,while connecting the clock input of the D flip-flop 128 to the CLK line66. By making the propagation delay to the DATA-IN line 64 be shorterthan the propagation delay to the CLK line 66, the D flip-flop 128 willalways capture and hold the correct data on the DATA-IN line at the timeof the clock transition. Thus, whenever a positive pulse is firstreceived on LINE 1 relative to LINE 2 (i.e., whenever the first pulse ofthe biphasic pulse pair is positive), causing the DATA-IN line to gohigh for the duration of the pulse as explained above, the D flip-flop128 captures such event as a high level, causing the buffered data line127 to latch high. Similarly, whenever a negative pulse is firstreceived on LINE 1 relative to LINE 2 (i.e., whenever the first pulse ofthe biphasic pulse pair is negative), causing the DATA-IN line to go lowfor the duration of the pulse as explained above, the D flip-flop 128captures such event as a low level, causing the buffered data line 127to latch low.

Note that the D flip-flop 128 need not be included as part of the lineinterface circuit 62 (although it could be), but rather is usuallyincluded in the state machine logic 94, or other processing circuitry,that receives and processes the incoming data.

Output data that is to be placed on the LINE 1/LINE 2 conductors at atime T2 after receipt of an input data pulse, is applied to the DATA OUTsignal line 74. Such data is then clocked into D flip-flop 132, causingthe data (a low or a high) to appear on signal line 134, at the end ofthe 44 μsec period set by the second timer 126. If the output data is a"1", i.e., a high level, such action triggers a third timer circuit 136,which times out after a prescribed time period, e.g., 1 sec. The timingout of the third timer circuit 136 resets the D flip-flop 132, therebycausing the signal line 134 to go low, thus forming an output data pulseof approximately 1 μsec width on the signal line 134. Such output datapulse is then inserted on the LINE 1/LINE 2 conductors by action of thecomplimentary switches M1 and M2, both of which are turned ON by the 1μsec output data pulse, with M1 pulling LINE 1 high, and M2 pulling LINE2 low, for the duration of the output data pulse, e.g., 1 μsec. Thus, inthis manner, the output data pulse appears on LINE 1/LINE 2 at a timeT2, as shown, e.g., in FIGS. 6 and 8.

If the output data applied to the DATA OUT line 74 is a "0", i.e., a lowlevel, a low level is clocked in to the D flip-flop 132. This meansthere is no change on signal line 134, the third timer circuit 136 isnot triggered, and the M1 and M2 triggers are not turned ON. Thus, thereis no data pulse inserted on LINE 1/LINE 2, and the absence of such datapulse signifies the output data is a "0". By having an output data "0"be represented by the absence of an output pulse saves operating powerof the device. Saving operating power is always an importantconsideration for the type of implantable sensor/stimulator devicesdescribed herein.

The 1 μsec time period set by the timer circuit 136 is achieved, in theembodiment shown in FIG. 9, by designing an inverter circuit usingcomplementary transistors M5 and M6, where the NFET (M6) of the pair isdesigned to have long dimensions, as seen in Table 1. Such longdimensions, in combination with the capacitance of capacitor C3(nominally 3 picofarads), produces the desired 1 μsec delay. The typicaldimensions of the other transistors, M1, M2, M3, M4 and M5 included inthe circuit of FIG. 9 are also shown in Table 1.

As described above, it is thus seen that the present invention providesa means whereby implantable sensors or stimulators may be daisy chainedtogether over a common power/data bus using a minimum number ofconnecting conductors, e.g., two, and wherein each individual deviceconnected in the daisy chain is individually addressable from a commoncontroller unit connected by way of the common power/data bus to each ofthe implantable devices.

As also described above, it is seen that the invention providesindividual implantable sensors and/or stimulators that can transmitand/or receive power and data signals over a minimum number of signallines connected therebetween.

As further described above, it is seen that each such implantablesensor/stimulator device of the invention has an hermetically sealedpart and a non-hermetically sealed part, with electrical feed-throughmeans for making electrical connections between the hermetically sealedpart and the non-hermetically sealed part, and wherein the hermeticallysealed part encompasses electrical circuits for operating andcontrolling the device, and further wherein the non-hermetically sealedpart includes a sensor for sensing a condition or substance to which thedevice is exposed, electrical terminals or pads to which connectingconductors may be connected, and/or electrodes through which stimulatingcurrent pulses may be applied to surrounding tissue or body fluids.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

What is claimed is:
 1. An implantable sensor/stimulator comprising:acarrier having first and second pads thereon; first and second lineconductors; wherein said first and second line conductors attached tosaid first and second carrier pads, respectively; a rectifier circuitcarried by said carrier and connected to the first and second pads; saidrectifier circuit including means for generatingy an operating voltagefrom biphasic pulses applied across said first and second pads; a lineinterface circuit carried by said carrier and connected to the first andsecond pads, said line interface circuit includingdetecting means forserially detecting whether the biphasic pulses applied across said firstand second pads are a first phase or a second phase, a first phasecorresponding to a received data bit representing a binary "1", and asecond phase corresponding to a received data bit representing a binary"0", whereby an input data stream is received by the line interfacecircuit from biphasic data pulses applied between the first and secondline conductors, and hence, between the first and second pads attachedto the first and second line conductors, and transmission means forserially applying a monophasic pulse having a first or second amplitudeacross said first and second pads at a time in-between when the biphasicpulses are applied across the first and second pads, a first amplitudeof the monophasic pulse representing a binary "1", and a secondamplitude of the monophasic pulse representing a binary "0", whereby anoutput data stream may be transmitted by the line interface circuit ontothe first and second pads, and hence, onto the first and second lineconductors attached to the first and second pads; a sensor carried bysaid carrier, said sensor having means for sensing a specified parameteror substance and means for generating an analog output signal thatvaries as a function of how much of the specified parameter or substanceis sensed; a converter circuit carried by said carrier that converts theanalog output signal from said sensor to a digital sensor signalcomprised of a multiplicity of respective bits; state machine meanscarried by said carrier for defining address data corresponding to saidimplantable sensor/stimulator, and for receiving detected data from thedetection means and determining if said detected data corresponds to thedefined address data of said implantable sensor/stimulator, and if so,responding thereto by applying the digital sensor signal to thetransmission means of the line interface circuit, wherein the respectivebits of the digital sensor signal function as the output data streamthat is transmitted by the line interface circuit onto the first andsecond pads, and hence, onto the first and second line conductorsattached to the first and second pads; and means for hermeticallysealing said rectifier circuit, line interface circuit, convertercircuit, and state machine means.
 2. The implantable sensor/stimulatorof claim 1 wherein the analog output signal generated by the sensorcomprises an electrical current, and wherein said converter circuitcomprises:a current-to-frequency converter circuit which includes meansfor converting the electrical current from said sensor to a variablefrequency pulsed signal, the frequency of the variable frequency pulsedsignal being a function of the magnitude of the electrical currentgenerated by the sensor; and a counter circuit that counts the pulses ofthe variable frequency pulsed signal over a specified period of time toform a count signal, the count signal thereby comprising said digitalsensor signal, said digital sensor signal providing a measure of thespecified parameter or substance sensed by the sensor.
 3. Theimplantable sensor/stimulator of claim 1 further including a stimulatorcircuit and at least one pair of electrodes connected to said stimulatorcircuit, said stimulator circuit being coupled to the state machinemeans and having means responsive to command data received by the statemachine means from the detecting means of the line interface circuit forgenerating a stimulation pulse that is applied to the at least one pairof electrodes.
 4. The implantable sensor/stimulator of claim 1 furthercomprising a plurality of sensors, each generating an analog electricalcurrent as a function of a specified sensed parameter, and multiplexermeans for selectively connecting each of the plurality of sensors to acorresponding converter circuit as controlled by command data receivedvia the line interface circuit.
 5. An implantable medical devicecomprising:an hermetically sealed part containing electrical circuitry;said electrical circuitry performing a medical function selected fromthe group consisting of medical electrical stimulation, medicalelectrically mediated stimulation, medical electrical sensing, thecombination medical electrical stimulation and electrical medicalsensing, medical electrical-mediated release of medication at an implantsite and medical electrical-mediated release of medication at anon-implant site; a non-hermetically sealed part, the non-hermeticallysealed part including a first pair of terminals and a second pair ofterminals, wherein the first pair of terminals are electricallyconnected to the second pair of terminals; and feed-through means formaking electrical contact between each terminal of the first pair andsecond pair of terminals and a respective portion of the electricalcircuitry within the hermetically sealed part; wherein said first pairof terminals comprises a means for applying electrical power and data tothe electrical circuitry within the hermetically sealed part, as well asa means for receiving data from the electrical circuitry within thehermetically sealed part, and said second pair of terminals similarlycomprises a means for passing the electrical power and data received onthe first pair of terminals to a corresponding first pair of terminalsof another implantable medical device, whereby a plurality of saidimplantable medical devices may be daisy-chained together by connectinga pair of conductors between the second pair of terminals of oneimplantable medical device and the first pair of terminals of anotherimplantable medical device; said implantable medical device wherein theimplantable medical device comprises an implantable sensor that senses aparameter associated with living tissue within which the medical deviceis implanted, and wherein the hermetically sealed part of theimplantable medical device includes electronic sensor circuitry foroperating and monitoring said sensor, and further wherein the electroniccircuitry receives electrical signals through the faced-through meansthat power and control the operation of said sensor; said medical devicewherein the implantable sensor generates an electrical current having amagnitude that varies as a function of the sensed parameter, and whereinthe electronic sensor comprisesa rectifier circuit connected to thefirst pair of terminals, said rectifier circuit including means forgenerating an operating voltage from an electrical signal applied acrosssaid first pair of terminals; a line interface circuit also connected tothe first pair of terminals for detecting input data within theelectrical signal applied across said first pair of terminals, and forallowing output data to be placed within the electrical signal appearingacross said first and second pair of terminals; current-to-frequencyconverter means coupled to the sensor for converting the electricalcurrent generated by the sensor to a signal comprising a stream ofelectrical pulses, wherein the time interval between adjacent pulses ofsaid stream of electrical pulses varies as a function of the magnitudeof the electrical current; and a counter circuit for counting the numberof pulses that occur within the stream of electrical pulses within a settime period, the number of pulses thus counted providing a measure ofthe electrical current, the count derived by said counter circuit aftersaid set time period comprising output data that is coupled to said lineinterface circuit.
 6. A chain of serially-connected electronic devicesadapted for implantation in living tissue comprising:a plurality ofsubstantially identical electronic devices, each electronic deviceincluding a first pair of terminals, a second pair of terminals, meansfor receiving an input pulsed data-stream signal over the first pair ofterminals, means for performing a medical function selected from thegroup consisting of medical electronic stimulation, medicalelectronically mediated stimulation, medical electronic sensing, thecombination medical electronic stimulation and electronic medicalsensing, medical electronic-mediated release of medication at an implantsite and medical electronic-mediated release of medication at anon-implant site, and means for applying output pulses to the first pairof terminals representative of the specified function performed so thatthe output pulses are interleaved between pulses of the input pulseddata-stream signal; a control circuit having a first line terminal and asecond line terminal, means for generating the input pulsed data-streamsignal and applying said input pulsed data-stream signal between thefirst line terminal and the second line terminal, and means fordetecting any output pulses appearing between pulses of the input pulseddata-stream; a first line conductor connected between the first lineterminal of the control circuit and a first terminal of the first pairof terminals of a first electronic device, and a second line conductorconnected between the second line terminal of the control circuit and asecond terminal of the first pair of terminals of the first electronicdevice; and for each additional electronic device included in the chainof implantable serially-connected electronic devices, wherein anelectronic device in the chain that is closest to the control circuitcomprising a forward electronic device, and an electronic device in thechain that is next closest to the control circuit comprising a rearwardelectronic device a signal conductor connected between a first terminalof the second pair of terminals of the forward electronic device and afirst terminal of the first pair of terminals of the rearward electronicdevice, and a return conductor connected between a second terminal ofthe second pair terminals of the forward electronic device and a secondterminal of the first pair of terminals of the rearward electronicdevice; whereby the first electronic device is connected to the controlcircuit with just two conductors, and each rearward electronic device inthe chain of electronic devices is also coupled to the forwardelectronic device with just two conductors; said chain of implantableelectronic devices wherein the specified function performed by each ofthe plurality of electronic devices comprises sensing and measuring aparameter associated with the living tissue within which the chain ofdevices is implanted; said chain of implantable electronic deviceswherein the input pulsed data stream signal generated by the controlcircuit comprises a train of biphasic pulses, with each biphasic pulsein the train of pulses comprising a negative pulse followed by apositive pulse to represent one binary state, and a positive pulsefollowed by a negative pulse to represent the other binary state, andwherein each biphasic pulse is separated from an adjacent biphasic pulsein the train of biphasic pulses by a time period T1, and further whereinthe train of biphasic pulses is applied to the first pair of terminalsof each electronic device in the chain.
 7. The chain of implantableelectronic devices as set forth in claim 6 wherein the train of biphasicpulses comprises a sequence of biphasic pulses grouped in a frame, witheach biphasic pulse representing a data bit, wherein a first group ofdata bits within the frame comprises an address, a second group of databits within the frame comprises an operational code, and a third groupof data bits within the frame comprises data.
 8. The chain ofimplantable electronic devices as set forth in claim 7 wherein the thirdgroup of data bits includes a first subgroup of data bits representingop code bits, and a second subgroup of data bits representingtransmitted data.
 9. The chain of implantable electronic devices as setforth in claim 8 wherein the data frame further includes at least oneadditional biphasic pulse representing a parity bit.
 10. The chain ofimplantable electronic devices as set forth in claim 8 wherein the dataframe further includes a first biphasic pulse representing a start bit,and a last biphasic pulse representing a stop bit.
 11. The chain ofimplantable electronic devices as set forth in claim 6 wherein theoutput pulses generated by each electronic device comprise output pulseshaving a variable amplitude, and wherein each electronic device includesmeans for interleaving the output pulses in the data stream of biphasicpulses at a time period T2 from a prior biphasic pulse, where T2 is lessthan T1.
 12. The chain of implantable electronic devices as set forth inclaim 11 wherein the output pulses assume a first amplitude to representa first binary state, and a second amplitude to represent a secondbinary state.
 13. The chain of implantable electronic devices as setforth in claim 11 wherein the output pulses assume a first amplitude torepresent a first binary state, and are absent, i.e., assume a zeroamplitude, to represent a second binary state.
 14. The chain ofimplantable electronic devices as set forth in claim 11 wherein theoutput pulses comprise monophasic pulses.
 15. The chain of implantableelectronic devices as set forth in claim 11 wherein the output pulsescomprise biphasic pulses.
 16. A method of sending power and control datato, and receiving transmit data from, a plurality of implantable medicaldevices, each connected to a two-conductor cable, comprising:(a)generating a biphasic pulse train of biphasic pulses at a regularrepetition rate of F1 pulses per second (pps), wherein each biphasicpulse in the pulse train has a width T_(W) and is separated from anadjacent pulse in the pulse train by a time separation distance of T1,where T_(W) is <<T1, and T1=1/F1; (b) modulating the pulses of the pulsetrain to represent data bits, wherein a biphasic pulse which goespositive first and then negative comprises a first binary state, e.g., alogical one, and wherein a biphasic pulse which goes negative first andthen positive comprises a second binary state, e.g., a logical zero; (c)arranging the biphasic pulses of the pulse train in data frames of Ndata bits each, wherein a first subset of the N data bits compriseaddress bits, a second subset of the N data bits comprise an operationalcode, a third subset of the N data bits comprise data; (d) applying thedata frames to the two-conductor cable so that the data frames arereceived by each of the plurality of implantable medical devicesconnected to the two-conductor cable; (e) receiving and processing thedata frames within each medical device by:(1) splitting the receivedbiphasic pulses into first and second signal paths, (2) rectifying andfiltering the biphasic pulses received in the first signal path tocreate an operating voltage used to operate the medical device, (3)demodulating the address bits received in the second signal path of eachdata frame to determine if the address bits match a predeterminedaddress stored in the medical device, and (4) if the address bits matchthe predetermined address, demodulating the operational code and datareceived in the second data path; and (f) sending transmitted data froma medical device for which a match exists between the address bitsreceived and the predetermined address by:(1) generating an output datapulse having a width <<T1, the output data pulse having a firstamplitude, e.g., a maximum amplitude, to represent a first binary state,and a second amplitude, e.g., a minimum amplitude, to represent a secondbinary state, and (2) applying the output data pulse to thetwo-conductor cable at a time in the data frame that is inbetween thebiphasic pulses of the N data bits that define a data frame.
 17. Themethod of claim 16 wherein the first amplitude of step (f) (1) comprisesa non-zero amplitude, and the second amplitude of step (f) (1) comprisesa zero amplitude, whereby the presence of an output data pulse having adiscernable amplitude at the time in the data frame that is inbetweenthe biphasic pulses of the data bits in the data frame represents thefirst binary state, and the absence of an output data pulse representsthe second binary state.